2+ feet and you should be fine.

In other words do not place anything on top of or around the speakers, especially any computer CRT monitors or TV sets, your mixer and your hard drives. A fake flower in a vase without water would make a nice touch? But then again so would a few beers??

Soggy speakers like Jimi Hendrix but then again, he's dead.
Ms. Remy Ann David

I don't know the math, but the actual magnetic field around a speaker magnet drops off fairly quickly beyond a foot or so. (Something about the inverse cube root? Distance vs. power? I can't recall the theorem now, but it's out there if you google it...)

This is why magnetic weapons and comic-book villians really don't pose much of a threat; the actual power needed to create dangerous magnetic fields - strong enough to do REAL damage over any kind of distance - is quite huge. The kind of permanent magnets found in speakers are fine for the job as long as you're right up next to them, but as soon as you move beyond a few inches away, the power/pull quickly drops off.

You should be fine at a foot or two, remember that the HD is shielded as well, for everyday magnetic interferance thats normally all around us. The most interference I've ever had to deal with were reel to reel tapes put directly against a speaker box (which showed surprisingly LITTLE erasure, actually) and more recently, a center/dialog speaker that caused some interference with my video monitor. We moved it 2 inches further up, and the problem went away.

Better safe than sorry, of course.